Variable Air Volume (VAV) temperature control systems control the temperature within a room by modulating the amount of cool (or warm) air that is blown into the room by the heating and air conditioning system. A pressure dependent VAV temperature control system uses a temperature control loop to accomplish a temperature control algorithm. The temperature control loop attempts to maintain a room temperature setpoint by using readings from a room temperature sensor to control a damper in a VAV box. Opening and closing the damper in the VAV box varies the airflow through the VAV box into the room.
One drawback to pressure dependent VAV temperature control systems is that a central Air Handling Unit supplies air to many VAV boxes and when the damper of one VAV box is opened or closed to adjust the temperature in its associated room, the airflow into the other rooms will be affected. This can cause the temperature control loops to “fight” each other, as a temperature change in one room will likely cause a flow change in the other rooms. As a result of the flow changes in the other rooms, the temperatures in those rooms will also change and as the VAV boxes in those rooms react to the temperature changes, they will cause more flow changes and the cycle is repeated. Accordingly, pressure dependent VAV control systems can be inefficient for controlling a heating, ventilation and air conditioning (HVAC) system within a sufficiently large building having multiple rooms.
A pressure independent VAV control system overcomes the problem of temperature control loops “fighting” each other and thus can be more efficient for controlling HVAC system in large buildings. A pressure independent VAV system uses two basic control loops to accomplish the temperature control algorithm. These two basic control loops include a flow control loop for maintaining a specified airflow to the room and a temperature control loop for adjusting the airflow setpoint (specified airflow) of the flow control loop based upon the room temperature. A pressure independent VAV control system thus maintains the specified airflow into the room regardless of pressure changes caused by airflow changes in other rooms.
However, pressure independent VAV control systems also have associated drawbacks. For example, one problem with a typical flow control loop in pressure independent systems is that it is very difficult to provide stable airflow into the room without excessively modulating the damper. The excessive modulation of a damper eventually causes the damper to fail and require replacement. System downtime and maintenance expenses incurred due to failed dampers are common problems in the temperature control industry.
Another drawback associated with a typical flow control loop in a pressure independent VAV control system is the effect of turbulent flow in the air ducts of the system. Electronic sensors can accurately measure instantaneous flow in the air ducts, but the turbulent flow can cause the resulting signals to be very noisy. If the noise in the signals is not filtered in some manner, the flow control loop will react to the peaks and valleys of the signals. As a result, the dampers in the VAV boxes will be continuously adjusted in response to the peaks and valleys in the signals, instead of being adjusted in response to what really affects the room temperature, which is the average airflow into the room over time.
Yet another problem associated with the typical flow control loop of a pressure independent VAV control system results from the inherent non-linear response curve of each damper within each VAV box and from the fact that the electric motors which move these dampers can be damaged by repeated short pulses of current which attempt to move the damper a very short distance. If the pulse is short enough, the motor may not build up enough torque to overcome the static friction needed to move at all. If the damper doesn't move, the flow conditions won't change so the control system will give it another short pulse. If no additional protection is taken, the control system can pulse the motor indefinitely and damage the motor.
To prevent this potential for indefinite pulsing, a minimum run time of, say, one second is typically provided by the motor control circuitry. If the damper controller turns the motor on for even a brief time, the motor control circuit will run the motor for at least one second, ensuring enough torque is developed to actually move the damper. This will, in turn, produce a change in the airflow. If less than a one second movement was needed to bring the flow to setpoint, this one second minimum run time can drive the damper too far and result in a flow reading on the other side of the setpoint. Without additional protection, this would result in the controller giving a short pulse to the damper to move it in the other direction.
The minimum run time would again cause the damper to overshoot the setpoint, and the system could cycle indefinitely, causing excessive wear on the damper and the motor. To prevent this, a deadband is typically set so that the dampers do not move if the measured flow is within a specified tolerance of the setpoint. To prevent cycling, this deadband must be greater than the change in flow caused by a one second damper movement. The “worst case situation,” where a little damper movement produces a large change in airflow, typically occurs about mid-range on the damper response curve. In order to avoid cycling in this worst case situation, the flow control loop needs a deadband which is at least as wide as the flow change produced by a one second damper movement at this point.
However, a wide deadband produces inaccurate control at the low and high end of the damper response curve. The low end of the damper response curve is particularly problematic, because of indoor air quality concerns. A temperature control system not only modulates the airflow to keep the temperature in the room at a comfortable level, but also provides the minimum fresh air ventilation for the room. Therefore, a low flow rate and a very wide deadband may cause the airflow control loop to maintain a flow rate that is below the minimum ventilation flow, and cause a problem for indoor air quality.
While the aforementioned problems affect the flow control loop in pressure independent VAV systems, the temperature control loop in such system may also suffer from problems that affect its performance. The temperature control loop typically uses a traditional proportional, integral and derivative (PID) control algorithm to adjust the flow setpoint based upon the temperature in the room. For stable control, low gains are needed for the PID components, but for accurate control high gains are needed. Finding the correct gains is often a difficult process (called “tuning” the control system) and the result is a compromise between stability and accuracy.
Accordingly, there remains a need for an improved pressure independent VAV control system that overcomes some or all of the problems mentioned above.